Functional Responses of Riparian Vegetation to Streamflow Diversion in the Eastern Sierra Nevada

Partial streamflow diversion due to the siting of hydroelectric generating plants may increase the incidence and degree of water stress in riparian vegetation. This study, conducted on Bishop Creek in the eastern Sierra Nevada (California), compared the water relations of riparian vegetation on paired undiverted (natural flow) and diverted (low flow) reaches. Riparian plants on diverted reaches had reduced stomatal conductance and water potential compared to plants on undiverted reaches in a dry year, but not in a high—runoff year. Juvenile plants on diverted reaches had reduced stomatal conductance and lower midday water potentials relative to surrounding mature trees, a trend that was not observed on undiverted reaches. Predawn water potentials were uniformly high in all species and sites, but low midday water potentials (—1.2 to —1.5 MPa) were observed for most species on diverted reaches. Plants on diverted reaches possessed significantly smaller, thicker leaves and a reduced total leaf area relative to trees on streamside reaches. Reduced community leaf area and effective stomatal control of water loss may allow riparian corridors on diverted reaches to retain their canopies in low—runoff years. However, a long—term consequence of streamflow diversion may be selective mortality of juvenile plants because of the elimination of floods and high flows.